Method of producing leaded steel

ABSTRACT

High lead content of uniformly dispersed finely divided particles is obtained in cast steel through a programmed addition of lead to molten steel in a vessel and control of other process factors, such as temperature, agitation and slag cover, affecting dissolution and retention of lead in molten steel. In the addition of lead, an initial quantity is introduced to the steel, large with respect to the time during which it is added as compared with the total lead added relative to the total time span of lead addition, and subsequently additional led is added, the total sufficient to produce a lead content in the steel greater than 0.15 percent by weight and no greater than 0.26 percent by weight. Preferably the total lead is introduced as three to five discrete additions at time intervals of one to fifteen minutes.

. States Patent Thomas et al.

[ METHOD OF PRODUCING LEADED STEEL [75] Inventors: Jerry D. Thomas, Parma; Cecil B. Griffith, North Royalton, both of Ohio [73] Assignee: Republic Steel Corporation,

Cleveland, Ohio [22] Filed: June 19, 1972 [21] Appl. No.: 264,012

[52] US. Cl. 75/129, 75/61 [51] int. Cl. C22c 33/00 [58] Field of Search 75/129, 123, 61

[56] References Cited UNITED STATES PATENTS 2,182,758 12/1939 Harder 75/123 2,182,759 12/1939 75/123 2,854,716 1 10/1958 Funk 75/129 2,947,622 8/1960 Tenenbaum 75/129 3,141,767 7/1964 Funk 75/129 3,545,960 12/1970 McClellan 75/129 3,567,204 3/1971 And0.... 75/61 3,573,895 4/1971 Ostberg 75/61 3,664,826 5/1972 Kraemer 75/129 3,671,224 6/1972 North 75/129 Primary Examiner-4.. Dewayne Rutledge Assistant Examiner-*Peter D. Rosenberg Attorney, Agent, or Firm-Watts, Hoffman, Fisher & l-leinke Co.

[57] ABSTRACT High lead content of uniformly dispersed finely divided particles is obtained in cast steel through a programmed addition of lead to molten steel in a vessel and control of other process factors, such as temperature, agitation and slag cover, affecting dissolution and retention of lead in molten steel. In the addition of lead, an initial quantity is introduced to the steel, large with respect to the time during which it is added as compared with the total lead added relative to the total time span of lead addition, and subsequently additional led is added, the total sufficient to produce a lead content in the steel greater than 0.15 percent by weight and no greater than 0.26 percent by weight. Preferably the total lead is introduced as three to five discrete additions at time intervals of one to fifteen minutes.

13 Claims, 7 Drawing Figures PAIENI sEPx 1 Ian SHEEI 1 OF 7 MOLTEN STEEL TAPPED ("0 VESSEL.

5L. A 6 C0 l EE APPLIED VESSEL PLACED IN sT/RR/Ne APPARATUS MULT/PLE D/SCEETE LEAD AUDIT/0N5 AT T/ME INTEEl AL5 57722 (NE DUE/N6 AND AFTER EACH LEAD A 00 7/ ON LEAD DISSOLUT/OA/ *1 ILEADED 57551. I 'FLOWS nv'ro I CONT/A/UOU5 cnsm/e MOLD LEADED STEEL CAST STEEL 6OL/D/F/E5 LEAD QEJECTED FEOM 5OLUT/ON.' FINE UNIFORM DISIQEES/ON (mo/550M150 LEAD SETTLED A Fig. l

PATENIEU SEPI 7 m4 SHEEI 2 BF 7 5 LEAD ADD/T/ONS 3 [LEAD ADDITIONS EAD ADDITION I LEAD ADD/770M ART SHEEI 3 OF 7 5 LEAD ADDITIONS KEY 25% Pb HEAT 207- Pb HEAT ./5 A Pb HEAT 4 a m .m u M .L F r T m n M 1 Mr M I rfia/ a PATENIEB 71974 L 0.// "/0 Pb HEAT A .07% Pb HEAT BK DENOTES LEAD ADDITION O DENOTES TAP OF FURNACE m m M n 0 hzmummu 5mm; 2 $25.28 33 TIME IN MINUTES D/SSOLUT/ON OF Leno IN 200 Le. Ham's OF 415/ 4/15 Leno 40050 A5 LEAD 5H0? WRAPPED/N STEEL SHEET AND THeuaT THROUGH SLHG covee o/= 4/5/ 4H6 HEAT AT 900F LEAD CONTENT W WE/GHT PERCENT PAIENIED 3.835.359

SHEET Q [1F 7 Qie DEMOTES LEAD ADDITION I Pb- /005 STEEL COMPACTED PELLET5 DEOPPED ONTO SURFACE OF Fl\/EPOUNO HEAT OF 4H8; FIVE ADDITIONS OF o./2/, Pb PE)? ADDITION.

O Pb SHOT COMPQCTED PELLETE DEOPPED ONTO SURFACE OF FIVE- POUND HEAT OF 4H8; FIVE ADQ/T/ NE OF o./2% Pb PEP ADO/T/ON.

A Pb SHOT WEAPPEQ //v PEOTECT/VE FOIL 4N0 THRUST THPoue-H SLAG COVE/2 OF F/vE-PouNo HEAT OF 4H8; Flt E AQD/T/ONE OF o,/2 Pb PEP Aoo/T/olv 2e I 24 .22 i .20

Y El

QEDG

THE D/fifiOLUT/ON 0F LEAD lN MOLTEA/ 4/5/ 4H5 57'EEL. USING THE METHOD OF MULTIPLE AUDIT/0N5 0F LEGO SHOT.

II I? I3 I4 15 Fig. 4A

2% PA EOTECT/VE FO/L T/ONS.

SHEET 5 BF 7 WRAPPED w 0 ND ADDED as FIVE n00/T/o/v5 0F 06 FEB ADDITION WRAPPED IN PROTECTIVE 5 POWDER ADDED A5 FIVE ADDITION-5 DF 0-/ PER ADD/T/ON 0 Li COMPOU FOIL DENOTES LEAD ADD! A 06 PATENIEBSEPI mm .hzmummq klQmR Z .hzmhzmu Q mq I T/ME IN M/NUTES THE D/55OLUT/ON 0F LEAD IN MOLTEN 43/51 4H5 STEEL USING THE METHODOF MULTIPLE ADU/T/OA/S OF THE LEAD COMPOUNDS, LEAD 5ULPH/0E 4ND L/TH/UM LEAD.

F1g.4B

PAIENIEDSEPI 71974 SHEET 7 0F 7 A F/FTEEN 0 04% mama/vs OF Leno SHOT MAP/ ED IN PPOTECTIVE FOIL.

0 HEAT WHICH W45 EEPET/T/ON 0F Aeol/e. ale DENOTE5 LEAD ADDITION.

TIME IN MINUTE-5 w m m m m w. w m RZMUQMQ RISER 2 HIE-20w Q mq LEAD DIESOLUT/ON IN 4/5! 4H6 5TEEL A5 EXH/B/TEO WHEN MAKING NUMEEOU5 5MALL LEAD AUDIT/0N5 TO MOLTEN STEEL. AT 2900'F.

Fig. 6

METHOD OF PRODUCING LEADEI) STEEL BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to leaded steel and processes for producing leaded steel.

2. Prior Art Current practices of leading steel to obtain free machining properties apparently originated in the late 1930s and early 1940s as evidenced by U.S. Pat. Nos. 2,182,758; 2,182,759; 2,197,259; 2,215,734; 2,236,479; 2,259,342 and 3,141,767, and by United Kingdom Pat. No. 519,572. These patents basically relate to the addition of lead to different steels, certain means of marking the lead addition, and the forms of the lead additive. As disclosed therein, lead is added to steel in finely divided form either in a vessel, a mold or in the pour stream as the steel is poured into a mold. A further approach, in which a stream of molten steel is fed into a bath of molten lead and allowed to overflow is disclosed in the somewhat later United Kingdom Patent No. 894,019. Somewhat similarly, lead added to a pool of steel that fills molds in bottom pouring equipment is disclosed in US. Pat. No. 2,854,716 in an attempt to overcome objectional features and results of prior art. Notwithstanding a stated desideratum in the prior art of obtaining a highly uniform submicroscopic dispersion of a significant level of lead in steel, the fact is that leaded steels have suffered from substantial, undesirable, macroscopic, lead segregation, especially at high lead contents. This results in non-uniform distribution of the lead content in cash ingots or billets, both transversely and longitudinally, and disruption of the ingot or billet surface by large particles of lead. Such ingots require additional bottom discard practice and such steel produces inferior surface quality in machined products.

In the co-pending application of Donald F. North et al., Serial No. 64,230 filed Aug. 17, 1970 now U.S. Pat. No. 3,671,224 and assigned to the assignee of the present invention, methods are disclosed and claimed for achieving a uniform submicroscopic dispersion of lead at levels up to 0.15 percent, representative of the solubility limit of lead in steel attainable by the therein disclosed process. Yet, a commercial need for comparable uniformity and small particle size of lead in steel at somewhat higher levels is not satisfied.

SUMMARY OF THE INVENTION The present invention is an improvement over that of North et al. referred to above, in that it overcomes the shortcomings of the previously known processes in the manner of North et al. and assures the production of steel in which the lead content in cast billets or ingots is essentially uniformly dispersed and finely divided, yet in addition achieves a significantly higher level of lead content. As in the North et al. process, the present process comprises a combination of steps that make effective use of solubility of lead in steel at temperatures above the liquidus. Upon solidification of the steel, the dissolved lead is rejected, resulting in the formation of a uniform, fine, dispersion of lead in the steel. By assuring that essentially the entire lead content in the molten steel is in solution, large particles and macrosegregation of lead are avoided. As a result, additional bottom discard practice (elimination of lower portions of ingots that contain excess lead) is avoided, ingot loss from gross segregation and large lead particles is eliminated, and the resulting leaded steel product meets critical surface requirements.

In accordance with the present invention, programmed lead additions made under specific process conditions materially increase the level of lead solubility over that heretofore obtainable with a single addition (from the previous .15 percent lead of the North et al. process to a presently obtainable 0.24 to 0.26 percent by weight) without disadvantageous gross segrega tion of lead or appearance of large lead particles in the steel characteristic of typical prior art processes that provide a high lead content. This higher lead solubility and the improved leaded steel product having highlead content, uniformly and finely dispersed without macrosegregation, is obtained by (a) adding lead to the steel in a vessel from which leaded steel containing essentially only dissolved lead is subsequently removed while molten to a casting mold, (b) establishing a minimum temperature of the molten steel at the time the lead is added that is significantly above the liquidus temperature of the unleaded steel, (0) introducing the lead to the steel by adding an initial quantity large with respect to the time for addition as compared with the total lead added relative to the total time span of lead additions and subsequently adding additional lead to reach a solubility level between 0.15 and 0.26 percent lead by weight, and preferably by adding the lead to the steel in a plurality but relatively limited number of discrete quantities or additions with a significant time interval therebetween, and also between the last addition of lead to the steel and the casting of the steel, (d) adding a limited excess of lead over the target quantity desired in the steel, (e) stirring the steel during lead dissolution, and (f) utilizing a slag covering over the steel during lead dissolution.

In addition to the above, other features enhance the solubility of lead or increase the efficiency of the pro cess. These features include (a) the use of finely divided lead particles or lead-containing particles to increase the surface area for dissolution, (b) the use of a relatively viscous slag to efficiently retard vaporization of dissolved lead, and (c) establishing a reasonably fast rate of solidification of the steel after casting.

A principal object of this invention is to achieve an increased level of dissolved lead in steel in a commercially practical process for producing cast leaded steel that assures a uniform dispersement of small finely divided lead particles and an absence of macrosegregation of lead and surface or subsurface streaks or blobs of lead.

Further objects and advantages of the invention will become apparent from the detailed description that follows.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a flow diagram illustrating both a general method of adding lead to steel in accordance with the present invention and a more specific process for the continuous casting of steel;

FIG. 2 is a diagrammatic illustration showing apparatus used in a process of FIG. 1; and,

FIGS. 3, 4A, 4B, 5 and 6 are graphs showing the content of dissolved lead in molten steel as a function of time under various conditions.

DETAILED DESCRIPTION In accordance with a preferred mode of practicing the process of the present invention, molten steel from a furnace, such as a BOF, electric arc furnace, or the like, is tapped to a vessel, such as a ladle, at as high a temperature as mill practice permits. A slag cover is applied to the steel in the vessel and the vessel is placed in a stirring apparatus such as an induction stirrer of a degasser, and the steel is stirred. Lead in divided form is added to the steel during stirring in multiple additions at significant intervals. The total quantity of lead added through the multiple additions is greater than the quantity desired in the final product by an amount at least as great as the anticipated lead losses, as through volatilization, occlusion in the slag and refractory materials and settling. The amount and form of lead added, the number and size of the lead additions, the time interval between additions, the stirring, slag cover and melt temperature, are all controlled to obtain desired results. Each of these aspects of the preferred method are considered in detail below.

Amount of Lead Added The present process relies upon and utilizes the solubility of lead in molten steel and the insolubility of lead in solidified steel to obtain a uniform dispersion of fine particles of lead in cast, i.e., solidified, steel. In the preferred practice of this process, a target or aimed lead content in the steel is selected consistent with a determined solubility limit of lead in the steel at the temperature when added to assure that the lead contentatll 2,854,716. While process variables in the present invention affect the solubility level attainable, i.e., the maximum target amount permissible if macrosegregation and large particle size of lead is to be avoided, a significantly higher upper limit of lead content for practical purposes has been found to be 0.24 to 0.26 percent by weight. Thus the present invention is applicable to the attainment of a lead content in steel of between 0.15 percent and 0.25 percent by weight, the process of a single lead addition being suitable to 0.15 percent but essentially inapplicable therebeyond regardless of the excess lead introduced in the one addition.

If a target amount of lead within the above-specified range is to be obtained in the cast steel, the actual lead added during the process must be greater than the target amount. The amount greater depends upon the level of lead desired within the 0.15 to 0.25 percent range, as well as upon process variables, such as stirring, temperature, etc. Where the process variables are controlled consistent with the entire process contemplated herein, the recovery in solution of the lead added to the steel ranges from a low of about 42 percent at the maximum solubility level of this process of 0.25 percent lead (thus requiring an excess of 140 percent over the target amount) to 56 percent at a solubility level of 0.18 percent (requiring an excess of 77 percent over the target amount). Experimental data illustrative of the above and showing the total amount of lead added to steel to provide the indicated amount actually obtained, based on 200 pound leaded steel heats, is set forth in Table I.

TABLE I THE METHOD OF MULTIPLE LEAD ADDITIONS USED IN MAKING 200 POUND LEADED STEEL HEATS No. of Lead Final Heat Additions Total Pb Added Desired Ingot Lead Grade Per Heat to Melt Pb Content Pb Recovery,

target level will have been in solution at the time of casting and will be precipitated in a uniformly distributed fine dispersion upon solidification. The actual amount of lead added to the steel is selected to assure that the target amount is obtained at the time of casting. Thus, the actual amount comprises the target amount plus an excess, based on anticipated lead losses during and after lead addition and prior to casting, due essentially to volatilization, occlusion of lead in the refractory container and the covering slag, and settling of undissolved lead. Notwithstanding the published experimental determination that the solubility limit of lead in low carbon (less than 0.10 percent carbon) steel may be as high as 0.43 percent, only a 0.15 percent dissolved lead content level has been attainable under practical conditions with the North et al. process where superior particle size and lead distribution is obtained. Higher levels obtained commercially have been at the expense of particle size and uniformity of distribution. Compare, for example, the slightly higher percentage of lead and the greater range of lead content disclosed in the aforementioned Funk et al. US. Pat. No.

A. E. Lord and N. A. Parlec, The Solubility of Lead in Liquid Iron," Trans. AIME, 218 (1960) 644-646.

By introducing a greater amount of lead in the initial addition than in subsequent ones, a greater efficiency can be obtained and it is believed that somewhat less excess over the target amount can be used than indicated in the table. Conversely, where process condi' tions in practice may not be controllable to establish near optimum procedures, a somewhat greater excess of lead over the target amount may be required, up to 1 percent lead by weight, but therebeyond the excess becomes ineffective and detrimental to the ability to cast the leaded steel without including undissolved, segregated, lead.

The excess lead over the target amount added to the steel melt takes into account lead losses in the molten steel bath which occur during handling and casting. The lead losses result in primarily five ways: (1) lead volatilization, (2) lead oxidation forming volatile lead oxides, (3) lead taken into the refractories, (4) lead taken into the slag that covers the melt, and (5) lead that settles without dissolving, especially as the solubility limit is approached. Of course, excesses of lead over the amounts needed to compensate for the above losses in obtaining the desired level of dissolved lead can be added if subsequently separated or purposefully allowed to escape so as not to become a part of the steel that is cast, but such excess represents an unnecessary inefficiency rather than a required excess and necessary part of the process.

The lead additions specified herein are applicable to steel alloys typified by AISI types 1020, l 1 18,41 18 and 4140, among others. It will be appreciated that the upper limit of lead that can be attained in accordance with this process, based on actual solubility, is influenced by differences in the steel composition, because of the effect of certain elements upon the solubility of lead.

Number and Size of Lead Additions and Time Interval Therebetween As shown in Table I above, percentages of dissolved lead are obtainable in molten steel, through plural additions of lead, beyond the maximum solubility level of 0.15 percent lead by weight obtainable under the practical limits of commercial process conditions through a single lead addition. Two to about additions are acceptable but in any event fewer than additions are contemplated. For percentages of lead content of between 0.20 to 0.25 percent by weight, three to five lead additions are most satisfactory, while two lead additions are acceptable for solubility levels of between 0.15 and 0.20 percent. Thus, to obtain 0.25 percent by weight lead by adding a total amount of lead equal in weight to 0.60 percent of the steel, it is advantageous to make three additions of lead equal to 0.20 percent by weight, four additions equal to 0.15 percent by weight, or five additions equal to 0.12 percent by weight, the amount specified in each instance being greater than the target level only by an amount necessary to compensate for the anticipated lead losses as discussed above. For even greater efficiency, the amount of lead in the initial addition should be slightly greater than in the subsequent additions. No single addition, however, should be greater than 0.35 percent of the melt by weight to avoid introducing excess lead that will merely settle out without any beneficial effect on the dissolution rate. This limit is slightly higher but generally consistent with the determination disclosed in said North et al. co-pending application, of the maximum amount of lead to be added for a single addition.

By dividing the total amount of lead to be added into relatively few, e.g., three to five, additions, a greater solubility efficiency is obtained because the quantity of lead added initially is sufficient to at least seemingly supersaturate the melt and form a suspension of small lead particles therein. The subsequent additions of lead each replenish that which has dissolved, so that the solution is highly saturated throughout a large part of the process.

The use of a larger number of smaller lead additions, such as upwards of 10 to 15 or more, is disadvantageous from the standpoint of requiring excess time to reach a given solubility level as compared with a fewer number of larger additions, for a total amount of lead to be added, and as high a solubility level is typically not obtained.

While currently viewed as disadvantageous from a practical standpoint, a controlled variable rate of continuous lead addition is regarded as equivalent to discrete lead additions in the broad sense, if programmed to produce the same general effect of initially establish- The time interval between discrete lead additions can vary from I to 15 minutes and typically will vary between 1 to 7 minutes. The specific time depends upon the utilization rate of the lead, i.e., the rate at which it dissolves, and the rate at which losses occur. The solution rate depends upon factors such as the stirring rate, melt temperature and cooling rate, and others, and the rate of lead loss depends upon the slag cover, the solubility of lead in the refractory of the vessel containing the melt, and the like. While a somewhat longer period of time between lead additions than specified can be used, especially if heat loss is compensated for or minimized, a loss of efficiency, i.e., a delay or increase in total time without an accompanying benefit, will result. This is appreciated from the fact that the solution reaction of lead in molten steel proceeds rapidly with stirring for a limited time such as one to two minutes. Thereafter, the solution reaction becomes balanced by a lead vaporization reaction and further time before the next lead addition does not produce an increased lead level. The solution reaction time is increased by the use of a slag layer over the molten steel. To achieve efficiency, then, the time between lead additions should be selected to produce as high an average solution rate as possible.

Considering both the size of the lead additions and the frequency, a single addition of a total amount of lead fails to provide adequate time during which the lead remains suspended for dissolution before settling and volatilization take their toll, whereas small amounts of lead introduced at close intervals fail to saturate the solution and result in a slower rate of dissolution. In both instances inefficiencies result and either a maximum lead level is not attained or excessive lead must be used with increased risk of attendant lead segregation in the final product. A lead content of 0.20 to 0.25 percent by weight lead is achieved in the preferred practice by utilizing three to five lead additions spaced apart one to seven minutes.

The graph of FIG. 3 depicts the lead content with re spect to the time from a first lead addition in the five 200 pound steel heats (A181 4118 steel) for which Table I tabulates certain data, and indicates the time interval between lead additions. The lower two curves are illustrative of the prior process disclosed in the aforementioned co-pending application, in which only a single addition of lead is made. The other three curves show the lead content at different times for melts having final lead levels of 0.18 percent, 0.20 percent and 0.25 percent. Additional details are included in the examples below.

Point in Process Where Lead is Added In the casting of steel according to this process, the steel is contained in a vessel such as a ladle prior to teeming. For a number of reasons, it is advantageous to add the lead to the steel in a vessel. One principal reason is that in the event the actual lead losses are less than anticipated or some settling of undissolved lead occurs, especially where the target amount of lead is near or at the solubility limit, excess undissolved lead will or can be segregated within the vessel and steel containing only dissolved lead can be removed from the vessel and cast. Other reasons are that in facilitates providing the steel at the proper temperature for the lead addition, the steel can be conveniently stirred before, during and after lead addition, and adequate time can be provided for dissolution of the lead in the steel prior to casting. Further, it is relatively easy and convenient to add lead into a vessel such as a ladle, an important consideration due to the high volatility of lead. Addition of lead to steel in'a vessel prior to casting also provides flexibility by which the present process is readily adapted to a variety of casting techniques, especially continuous casting.

Temperature of Steel When Lead Added To obtain leaded steel in a fine dispersion and uniformly distributed in accordance with this invention, the steel must be at least ten degrees Fahrenheit above the liquidus temperature of the steel. This limit is arbitrary to the extent that there is no sharp break in the solubility curve at that temperature, but is a realistic and necessary limitation from a practical standpoint in assuring adequate solubility. Optimum conditions for obtaining maximum solubility and maximum lead content require still higher temperatures above the liquidus. In accordance with this invention, optimum lead solubility in low carbon (i.e., less than 0.10 percent carbon) steel is obtained by adding lead to the steel bath with the steel bath at a temperature as high as mill practice will permit, which is seldom higher than 3,000 degrees Fahrenheit and typically 75 to 135 degrees Fahrenheit above liquidus. Thus, where the liquidus is 2,775 degrees Fahrenheit, the temperature of the melt at the time of lead addition would advantageously be 2,850 to 2,910 degrees Fahrenheit. This may be referred to as the optimum temperature for this steel and is that temperature of the melt necessary to attain the maximum solubility of lead in the molten steel, optimum ingot quality, and/or optimum casting practice.

In the present process, the lead will be in contact with molten steel prior to being cast, for a time of about five to 30 minutes, i.e., from the first lead addition to the start of casting. During this time, the steel is at a temperature substantially above the liquidus, especially in the process of continuous casting, where the steel temperature when introduced to the ladle .is about 50 to 150 degrees Fahrenheit higher than the temperature normally used for ingot casting.

Stirring The solution rate of lead in molten steel depends significantly upon stirring, i.e., mechanical mixing of the steel and added lead. To assure attainment of high dissolved lead contents as well as the dissolution of a large percentage of the lead actually added to the melt, stirring must be maintained for a period during and after each lead addition. Preferably, a stirring action is maintained throughout the period of time during which multiple lead additions are made and is continued for a time after the last addition. Where three to five lead additions are made one to seven minutes apart, a minimum total stirring time of five minutes is required and most desirably the stirring time will be fifteen to thirty minutes in commercial practice, assuming a generally constant, somewhat intense or vigorous, stirring rate. For maximum dissolution and efficiency, the stirring rate should be maximized. A suitable stirring rate will cause an obvious, visible, flow of metal in the vessel, which when created by induction, as in a small induction furnace, is sufficient to cause an upward bulging of the metal top surface. Stirring is continued after the last addition of lead, preferably for a period of time somewhat in excess of the -stirring time provided between lead additions, because of the greater difficulty in dissolving lead as the dissolved lead content of the melt increases and approaches the solubility limit.

In addition to the effect on the dissolution rate, the stirring of the melt cleanses the steel of undissolved lead in instances where some excess of lead may exist above the solubility limit. In such instances the undissolved lead is rapidly settled from the steel within the ladle or other vessel, assuring that with proper pouring or teeming techniques no undissolved lead will be removed with the steel.

Form of Lead Added This process contemplates that the quantity of lead comprising each addition be in a divided form to provide a high surface area-to-volume ratio and may be added in the form of small lead chunks, lead shot, leadsteel pellets, or lead-iron pellets, which increase the degree of immediate dispersion and decrease the time required for dissolution and dispersion. In general, a small lead particle size enhances the possibility of reaching maximum lead solubility and increases the solution rate.

No particular mechanism is required to introduce the lead and suitably lead particles may be wrapped in steel sheet and thrust through the slag cover on the bath. Alternatively, the lead additive can be in the form of steel capsules and be submerged beneath the bath surface, or lead shot or capsules can be poured through a tube in which the lower end is submerged beneath the bath surface. Lead shot or capsules can also be injected pneumatically or otherwise projected into the bath. Slag Cover A slag cover over the molten steel is necessary to inhibit the loss of lead through volatilization and serves to increase the dissolved lead content in the melt. By trapping lead vapor, the slag effectively increases the time available for lead dissolution and enhances the lead supersaturation of the melt. That is, the solution reaction can proceed for a longer time, until the vaporization reaction becomes predominant due to gradual dissipation of the higher lead partial vapor pressure initially built up between the melt and the slag cover after a lead addition. Any typical slag is satisfactory, for example, vermiculite or bauxite-lime, but the more viscous the slag the more effectively it traps lead vapor over the melt and reduces the rate of gaseous diffusion. The slag cover must be present immediately after lead addition, and preferably is present initially and pushed aside to expose a portion of the bath surface to which the lead is added. Tests have indicated that lead loss for heats without slag cover averaged 0.006 percent per minute. With slag cover, the loss is reduced to an average of 0.0013 percent per minute, measured when the bath was under heavy stirring from induction heating.

In commercial operations, with a slag cover, the lead content will remain relatively constant throughout a normal teeming operation. Leaded Steel Products The present process is contemplated for use with a composition of steel to which lead can be added and is especially useful in producing free machining steels. In a preferred process, the steel can be produced in billet form by continuous casting. Alternatively, the process is also applicable to the casting of ingots or the like.

The product produced is a steel alloy having enhanced machinability over a conventional steel without lead, but which will not have drawbacks, i.e., defects, as in conventionally leaded steel, which defects include surface and subsurface streaks of lead and blobs of lead. The product is characterized by an absence of macrosegregation of lead. Multiple lead additions are utilized to produce a product having a lead content of between 0.15 percent to 0.24-0.26 percent lead by weight in which the predominant lead particle size will be no greater than 10 microns and the maximum lead particle size will be no greater than 30 microns. In addition, the distribution of lead throughout the product is extremely uniform, generally varying no more than 0.01 to 0.03 percent lead by weight. While nonuniformity beyond this range has been experienced, it has been attributable to the presence of undissolved lead resulting from inadequate stirring, spill-over during pouring, low melt temperature, etc.,; i.e., a failure to observe the process requirements.

Exemplary Process and Apparatus in General The present process is applicable generally for producing leaded steel for casting ingots, billets and the like, and is especially applicable to continuous casting. A flow diagram illustrative of the general process embodying the present invention is shown in FIG. 11 of the drawings, with more specific steps utilized in a continuous casting process being illustrated in broken lines. Apparatus for practicing the process is shown in FIG.

With reference to the process and apparatus shown in the drawings, molten steel, such as low carbon steel, is tapped from a steel-making furnace (not shown) such as a BOF or other furnace, into a vessel, such as a ladle 10. In a commercial process, the ladle may typically hold 100 to 200 tons of steel, indicated at 12. A layer of slag 13 is applied to the steel 12 in a conventional manner, to retain heat, reduce fuming and lead loss after lead addition, and increase the level to which lead will dissolve in the steel. Approximately a two-inch layer of slag is adequate. Although not preferred, the slag may be applied to the steel in the vessel immediately after the first addition of lead.

The vessel, such as the ladle i0, is then transferred to stirring apparatus, such as a degasser 114, which provides an enclosed chamber MA and induction stirring apparatus MB. The temperature of the steel in the vessel is as high as mill practice will allow, at least 10 degrees Fahrenheit above the liquidus temperature of the steel at the time the lead is initially added, and typically at a temperature of between 2,850 and 2,910 degrees Fahrenheit for best results, considering the type of steel, the solubility of lead, and good casting practice. To achieve this temperature level, the temperature of the steel at the time it is tapped from the furnace may be approximately 3,000 degrees Fahrenheit. Vigorous stirring by induction is begun and is maintained at a substantially constant rate.

Lead to be added is in subdivided form, for example, in the form of small chunks, shot, or lead-steel pellets. When in finely divided form, such as shot, the lead may be advantageously wrapped in a steel sheet. The quantity of lead to be added depends upon the target amount desired in the cast steel. While the exact excess to be added over and above the target amount varies with process conditions, Table 1 above is a guide. As indicated therein, lead in the amount of 140 percent of the target amount was used to achieve the maximum solubility level.

To obtain a lead content greater than 0.15 percent, without gross segregation and with all lead in finely divided form, the lead is added in multiple additions with a time interval between. For example, to obtain a lead content of 0.25 percent by weight, a total of 0.60 percent lead by weight is added to the steel in three to five distinct additions of 0.20 to 0.12. percent by weight lead, respectively, one to seven minutes apart, with constant stirring during and after each lead addition, for a total time of five to 30 minutes. A more specific example is set forth below. The additional lead above the target amount allows for the anticipated loss of lead from the melt during addition and dissolution through lead volatilization, lead oxidation forming volatile lead oxides, lead taken into the refractories, lead taken into the slag, and settling. Lead is added by thrusting lead chunks or lead shot wrapped in steel sheet through the slag layer. The time interval between lead additions is selected so as to timely replenish the melt with lead as that previously added is dissolved and lost. After the addition of the total amount of lea-d contemplated and the completion of the stirring time, the desired solubility level is reached and the stirring is stopped.

Because the lead is added to the steel in a vessel, such as the ladle 10, any lead that does not dissolve in the steel melt, refractory or slag, or volatilize, but rather settles, can be retained in the vessel so that only steel containing dissolved lead is removed from the vessel and is cast. This is accomplished either through the pouring technique or through a particular vessel con struction. As shown in FIG. 2, the ladle is constructed to prevent any excess lead not dissolved in the steel from being teemed during the continuous casting process, by utilizing a nozzle 18 projecting upward from the bottom of the ladle a short distance so that the bottom of the ladle acts as a reservoir to retain any settled excess undissolved lead. A rod 20 within the ladle functions as a stopper for the ladle and is moved vertically to open and close the nozzle by a lever 22.

While the lead-containing steel can be cast into individual molds, in which case the lack of lead segregation eliminates the need to bottom crop the cast billets and results in a greater yield, the uniformity of lead content makes practical the continuous casting of high lead content steel. This continuous casting process is accomplished by teeming the steel containing only dissolved lead from the ladle 10 into a tundish 24, which provides a uniform flow of steel to a continuous casting mold 30. The steel in the tundish is covered by a protective layer of slag 25. A ventilation tube 26 is provided adjacent the top of the tundish to withdraw any volatilized lead. The tundish serves to provide a substantially uniform pressure head and, hence, flow rate of the steel, which flows from the tundish through a fused silica nozzle 20 to the continuous casting mold 30, where it is solidified and advanced as a continuous formed billet 32.

The continuous casting mold is typically a water cooled copper mold. A special low fusion point slag material 34 is utilized over the steel in the continuous casting mold to reduce lead fuming from the molten leaded steel poured into the mold and to lubricate the mold surfaces to avoid sticking of the solidified steel to the mold walls. A ventilation tube 36 is provided at the top of the mold 30 to withdraw any volatilized lead.

llll

Upon solidification of the steel containing the dissolved lead, the lead is rejected from solution as a fine, uniform dispersion within the steel. Results in Practice Examples Laboratory heats were prepared and lead additions made, which demonstrate experimentally that the present process produces high quality leaded steel, of essentially uniform lead content throughout, with a fine and uniform lead particle distribution.

A series of heats were made of five pounds and two hundred pounds in an induction furnace. The fivepound heats were made under an atmosphere of argon and the two hundred pound-heats were made under an atmosphere of air. The melt temperature in both cases was maintained at approximately 2,900 degrees Fahrenheit, each melt as completely covered with a fluid lime-bauxite slag in a ratio of 60 percent lime, 40 percent bauxite. The base composition of the heats was AISI 4118 steel, as follows, in percent by weight (all proportions herein being expressed in percent by weight of the steel melt unless otherwise noted):

C Mn Si P S Cr Mo Each five-pound heat was used to make one ingot 2% by 2 by 3 /2 inches and each 200 pound heat was used to make two 72 pound ingots 4% by 4 by 15 inches in dimension.

1. Form of lead Using five-pound heats prepared as above, lead was added to separate heats in different forms. In each heat, five additions of lead in the amount of 0.12 percent by weight of the melt were added, except in the case of lithium lead compound, where the amount of lead was 0.20 percent by weight. The lead was added to molten steel as (a) lead-1008 steel (AISI designation) compacted pellets, (b) lead compacted pellets, free lead shot wrapped in thin protective foil (steel sheet or aluminum), (d) lead sulfide powder wrapped in protective foil, and (e) lithium lead compound wrapped in protective foil. The lead additions were made to each melt over a period of time of approximately minutes with continuous induction stirring. The lead content at various times during the process, as determined by spectrographic analysis of melt samples (so-called pin samples) are shown in FIGS. 4A and 4B of the drawings. A lead content in solution of approximately 0.25 percent by weight was achieved in the trials shown in FIG. 4A. It can be seen from the graph of FIG. 48, that the lead content achieved through the use of lead sulfide powder leveled off at 0.20 percent, apparently due to a variation in stirring and temperature. The lead content of the melt to which the lithium lead compound was added went beyond the solubility limit because of the excess lead added over the 0.60 percent by weight of the other examples. As noted below in connection with tests showing the solubility limit, when the lead content exceeds 0.24 to 0.26 percent by weight, the lead is not in solution and does not appear in a finely dispersed uniform distribution in the solidified steel. Microprobe X-ray analyses and metallographic studies support the conclusion that the lead solubility limit in molten steel is approximately 0.25 percent by weight.

While a slight difference in the solution rate is shown by the slope of the curves in FIGS. 4A and 4B, believed due to differences in the induction stirring rate and temperature changes, the graphs indicate that lead can be added in a number of ways and still reach the lead solubility limit in a similar kinetic manner.

2. Solubility limit To determine the solubility limit of lead in steel, lead was added to two additional five-pound heats of AISI 4] 18 steel prepared above and at a temperature of 2900 degrees Fahrenheit. Dissolved lead content is defined for the present purpose as the lead content determined by spectrographic analysis from pin samples of the heats when the lead particles in the solidified steel are evenly distributed and all are essentially one to 10 microns in size. It has been found that once pin samples indicate a consistent lead content after a certain level of lead addition, a further increase in the lead content results in a loss of uniformity of lead distribution in the ingot, and the lead solubility limit in molten steel can be assumed to have been reached and surpassed.

A graph is shown in FIG. 5, indicating points at which lead additions were made to the two heats referred to above and showing the lead content at different times as determined by spectrographic analysis of pin samples. Ten additions of lead of 0.12 percent lead by weight, were made to one heat by thrusting lead shot wrapped in protective foil through the slag cover. In a second heat ten additions of 0.20 percent lead by weight were made in the same manner. In both cases, as shown by the curves on the graphs, the content of dissolved lead reached a plateau at 0.24 to 0.26 percent by weight. Eventually, due to successive lead additions, the lead content of the molten steel became greater than 0.26 percent by weight and inconsistent lead content was noted. When the lead distribution lost uniformity, the solubility limit had been surpassed.

3. Number and Size of Additions In another five-pound heat of AISI 4118 steel prepared as above and at a temperature of 2900 degrees Fahrenheit, lead was added in fifteen additions of 0.04 percent lead by weight, each addition being made at approximately 1 /2 minute intervals, over a 22 minute period. The lead was added in the form of lead shot wrapped in protective foil and the additions were thrust through the slag covering. The points of addition and the lead content as determined by spectrographic analysis of pin samples are shown on the graph of FIG. 6. A comparison of this graph with the graphs of FIGS. 3, 4A, and 48 indicates that the use of a substantially larger number of smaller lead additions than the three to five preferred reduces the rate of dissolution and also reduces the ultimate lead content obtained in the steel for a given total lead addition.

Three 200 pound heats of AISI 41 18 steel were prepared as set forth above in a 250 pound induction furnace operated at 50 kilowatts, which stirred the melt vigorously. A first lead addition was made to the steel, which was at a temperature of 2900 degrees Fahrenheit, in an amount of 0.20 percent by weight. The total lead addition made to each melt is shown in Table I above. The lead additions comprised lead shot wrapped in steel sheet and thrust through a slag cover over the melt. The second lead addition to each heat was added 2 /2 minutes after the first. In the 0.25 percent and 0.20 percent lead heats a third lead addition was made 2 /2 minutes after the second. The second and third additions in the 0.25 percent lead heat contained lead in the amount of 0.20 percent by weight of the melt. The second addition in each of the other two heats and the third addition in the 0.20 percent lead heat each con- SUMMARY With discrete multiple lead additions to a steel melt at significant time intervals, advantageously three to five additions, approximately one to seven minutes rained lead in the amount of 0.12 percent by weight. 5 The average solution rate was determined to be aplead Content beyond heretofore prficilcal proximately .035 percent lead per minute during the 5 percgnttgan g to i i p first three minutes. The total time required to reach the i welg Sma .pamc e final lead Content was between Seven and eleven mm and uniform distribution, under commercially feasible utes. The time of lead additions and lead content as a i fi h resulting product has .mcreasefj mafunction of time for each heat are shown on the upper .Chmeablhty wlthoilt loss m surfac? quality or umform' three curves in the graph of FIG. 3 and the final content 1%? ami f g? i the seel ffi lead of the lead in cast ingots for each heat is included in a e m acco? e procia'ss Table 1 above. Two additional curves are shown in FIG. C osed m the aforemennoned co'pendmg apphcauon' 3 illustrative of the prior art where a single lead addi- The methods described above and the parameters inti n was mad volved are applicable to steel making processes in 4, Di t ib ti f l d i th steel which it is desired to produce a leaded steel of high The lead content at various locations in three ingots q y- W111 be pparen t0 thOSe Skilled in the art r d d f fl d heats d th Seventythat varlations can and will be made to meet various two pound ingots produced from 200 pound heats of conditions and specific data has been provided herein AIS] 41 18 steel are shown in Table I]; to provide a guide for achieving variations that may be TABLE II Lead content at cross sections Ingot (4118 Steel) Total Pb Top Middle Bottom Outer Lead content added (peredge Center (percent by Weight No. of cent by M d M d- Mid- (2 loca- (3 1ocaweight) (pounds) additions weight) Edge point Center Edge polnt Center Edge point Center tions) tions) 0, 72 3 0. 60 0. 24 0. 27 0. 29 0. 24 0. 25 0. 26 0. 23 0. 25 0. 26 0. 20 72 3 0. 44 0. 2O 0. 20 0. 20 0. 19 0. 20 0. 20 0. 20 0. 20 0, 1s 72 2 0.32 0. 18 0. 18 0. 17 0. 18 0. 19 0. 17 0. 16 0.16 18 0. 19 5 5 0. 60 19 2O 21 0, 20 5 5 0. 60 22 22 15 0,15 5 15 0 60 .2 15 l6 Three 72 pound ingots were made, one from each of the first three heats of Table I. See also the top three curves of FIG. 3. Of the three five-pound ingots, the 0.19 and 0.20 percent ingots were made by making five equal additions of lead in the amount of 0.12 percent by weight and the 0.15 percent lead ingot was made by making 15 equal additions of lead in the amount of 0.04 percent by weight. Lead content in each instance was determined spectrographically to an accuracy of 10.01 percent by weight. The three 72 pound ingots were cut transversely at locations adjacent the top, middle and bottom and lead content was determined at three points along a radius from the center. The fivepound ingots were cut transversely only at one location and lead content was determined at two peripheral locations diametrically opposite each other and at three central locations. The lead was considered as nonsegregated (all resulting from dissolved lead in the melt) except for that in the 72 pound ingot having a lead content of 0.25 percent by weight, in which undissolved lead spilled from the vessel containing the steel near the end of the pour, accounting for the greater variation in lead distribution and the higher content of the lead at the center top portion of the ingot.

Except for the 0.25 percent lead ingot all exhibited a high degree of uniformity of lead distribution throughout. The generally lower lead content of the five-pound ingots was due to inadequate stirring and a discontinuous slag cover. In addition, the high number of small additions resulted in a still lower lead content in the 0.15 percent lead ingot. Due to process factors, the results obtained with five-pound heats were generally more erractic than with 200 pound heats.

desired. It will be appreciated, then, that while preferred embodiments of the invention have been described with particularity, the intention is to cover and secure all modifications, alternatives and equivalents within the spirit and scope of the invention expressed in the appended claims.

What is claimed is:

1. In a process of producing leaded steel having a lead content greater than 0.15 percent by weight and no greater than 0.26 percent by weight in the form of substantially uniform dispersion of finely divided lead particles, essentially all of the lead in said steel having been dissolved in the steel when molten, including the steps of adding lead to molten steel in a vessel while the steel is at least l0 degrees Fahrenheit above the liquidus temperature of the steel, providing a slag cover over the steel in the vessel and stirring the steel after the lead is added, introducing an initial quantity of lead to the steel and subsequently introducing additional lead, said quantity of lead being a greater percentage of the total lead added than any subsequent addition, and the total lead added being sufficient to produce a lead content in the steel greater than 0.15 percent by weight and no greater than 0.26 percent by weight.

2. In a process of producing leaded steel having a lead content greater than 0.15 percent by weight and no greater than 0.26 percent by weight in the form of a substantially uniform dispersion of finely divided lead particles varying in the amount present throughout major portions of the steel by no more than 0.03 percent by weight, the predominant size of said particles being no greater than about 10 microns in diameter and essentially all of the lead in said steel having been dis- V solved in the steel when molten, including the steps of adding lead to molten steel in a vessel while the steel is at least 10 degrees Fahrenehit above the liquidus temperature of the steel, providing a slag cover over the steel in the vessel and stirring the steel after the lead is added, introducing an initial quantity of lead to the steel and subsequently introducing additional lead, said initial quantity of lead being a greater percentage of the total lead added than any subsequent addition, the total lead added being sufficient to produce a lead content in the steel greater than 0.15 percent by weight and no greater than 0.26 percent by weight.

3. In a process of producing leaded steel having a substantially uniform dispersion of finely divided lead particles essentially all of the lead in said steel having been dissolved in the steel when molten, including the steps of adding lead to molten steel in a vessel while the steel is at least 10 degrees Fahrenheit above the liquidus temperature of the steel, providing a slag cover over the steel in the vessel and stirring the steel after the lead is added, the improvement comprising introducing the lead as at least two substantially discrete additions, each having a lead content greater than onefifteenth of the total amount of lead added and spaced apart in time by at least one minute, the total additions together containing an amount of lead sufficient to produce a lead content in the steel greater than 0.15 percent by weight and no greater than 0.26 percent by weight.

4. A process as set forth in claim 3 wherein the first of said at least two substantially discrete lead additions has a lead content at least as great as any subsequent lead addition.

5. In a process of producing leaded steel having a substantially uniform dispersion of finely divided lead particles varying in the amount present throughout major portions of the steel by no more than 0.03 percent by weight, the predominant size of said particles being no greater than about 10 microns in diameter and essentially all of the lead in said steel having been dissolved in the steel when molten, including the steps of adding lead to molten steel in a vessel while the steel is at least 10 degrees Fahrenheit above the liquidus temperature of the steel, providing a slag cover over the steel in the vessel and stirring the steel after the lead is added, the improvement comprising introducing the lead as at least two substantially discrete additions, each having a lead content greater than one-fifteenth of the total amount of lead added but no greater than 9.35 percent by weight of the molten steel and added lead and spaced apart in time by at least one minute, the total additions together containing an amount of lead sufficient to produce a lead content in the steel greater than 0.15 percent by weight and no greater than 0.26 percent by weight.

6. A process as set forth in claim 5 wherein the first of said at least two substantially discrete lead additions has a greater lead content than any subsequent addition.

7. In a process of producing leaded steel having a substantially uniform dispersion of finely divided lead.

particles essentially all of the lead in said steel having been dissolved in the steel when molten, including the steps of adding lead to molten steel in a vessel while the steel is at least degrees Fahrenheit above the liquidus temperature of the steel, providing a slag cover over the steel in the vessel and stirring the steel after the lead is added, the improvement comprising introducing the lead in at least three and no more than five substantially discrete additions none of which contains lead in an amount exceeding .35 percent by weight of the molten steel and added lead separating each addition from the next by a time interval at least one minute and no more than fifteen minutes, the total amount of lead added being less than 1 percent by weight of the molten steel and added lead.

8. A process as set forth in claim 7 wherein the first of said discrete additions has a lead content at least as great as any subsequent lead addition and the time interval between additions is no greater than seven minutes.

9. In a process of producing leaded steel having a lead content greater than 0.15 percent by weight and no greater than 0.26 percent by weight in the form of a substantially uniform dispersion of finely divided lead particles varying in the amount present throughout major portions of the steel by no more than 0.03 percent by weight, the predominant size of said particles being no greater than about 10 microns in diameter and essentially all of the lead in said steel having been dissolved in the steel when molten, the steps comprising containing molten steel in a vessel at a temperature of between 10 and degrees Fahrenheit above the liquidus temperature of the steel, providing a slag cover over the steel in the vessel, introducing an initial quantity of lead in divided form to the steel and thereafter introducing additional lead to the molten steel, said initial quantity of lead being a greater percentage of the total lead added than any subsequent addition, the total lead added being sufficient to produce a lead content in the steel greater than 0.15 percent by weight and no greater than 0.26 percent by weight, stirring the molten steel in the vessel during and after the addition of lead for a total time period of between five and thirty minutes, removing steel containing lead essentially only in the dissolved state from the vessel, and solidifying the steel removed from the vessel.

10. A process as set forth in claim 9 wherein the lead is introduced as at least two substantially discrete additions, each discrete addition having a lead content greater than one-fifteenth of the total amount of lead added but not greater than 0.35 percent by weight of the steel and spaced apart in time by at least one minute and not more than 15 minutes, and the total lead added being less than 1 percent by weight of the steel.

11. A process as set forth in claim 10 wherein the first 7 of said at least two substantially discrete lead additions has a greater lead content than any subsequent addition.

12. A process of adding lead to steel to assure a uniform distribution of only finely dispersed lead particles the largest of which are no greater than about 10 microns in diameter, including the steps of first adding lead to steel in a vessel and then removing the leaded steel from the vessel to a mold, such as an ingot mold or a continuous casting mold or the like, wherein the temperature of the steel is at least 10 degrees Fahrenheit above the liquidus temperature of the steel when the lead is added, two to 15 discrete additions of lead are made at time intervals of at least one minute, a cover layer of slag is provided over the steel in the vessel, the steel is stirred within the vessel for at least one minute after each lead addition and for a total time of at least five minutes, and essentially all of the lead in the steel removed from the vessel is in a dissolved state.

13. A process of continuously casting steel containing a uniform distribution of finely dispersed lead particles the predominant size of which is less than microns in diameter and the largest of which are no greater than 30 microns in diameter, including the steps of: containing molten steel to which lead is to be added in a vessel at a temperature at least 75 degrees Fahrenheit above the liquidus temperature of the steel; adding an initial quantity of lead to the steel in the vessel and subsequently adding an additional quantity, said initial quantity of lead being a greater percentage of the total lead added than any subsequent addition, the total lead added being sufficient to produce a lead content solidified shape of leaded steel. 

2. In a process of producing leaded steel having a lead content greater than 0.15 percent by weight and no greater than 0.26 percent by weight in the form of a substantially uniform dispersion of finely divided lead particles varying in the amount present throughout major portions of the steel by no more than 0.03 percent by weight, the predominant size of said particles being no greater than about 10 microns in diameter and essentially all of the lead in said steel having been dissolved in the steel when molten, including the steps of adding lead to molten steel in a vessel while the steel is at least 10 degrees Fahrenehit above the liquidus temperature of the steel, providing a slag cover over the steel in the vessel and stirring the steel after the lead is added, introducing an initial quantity of lead to the steel and subsequently introducing additional lead, said initial quantity of lead being a greater percentage of the total lead added than any subsequent addition, the total lead added being sufficient to produce a lead content in the steel greater than 0.15 percent by weight and no greater than 0.26 percent by weight.
 3. In a process of producing leaded steel having a substantially uniform dispersion of finely divided lead particles essentially all of the lead in said steel having been dissolved in the steel when molten, including the steps of adding lead to molten steel in a vessel while the steel is at least 10 degrees Fahrenheit above the liquidus temperature of the steel, providing a slag cover over the steel in the vessel and stirring the steel after the lead is added, the improvement comprising introducing the lead as at least two substantially discrete additions, each having a lead content greater than one-fifteenth of the total amount of lead added and spaced apart in time by at least one minute, the total additions together containing an amount of lead sufficient to produce a lead content in the steel greater than 0.15 percent by weight and no greater than 0.26 percent by weight.
 4. A process as set forth in claim 3 wherein the first of said at least two substantially discrete lead additions has a lead content at least as great as any subsequent lead addition.
 5. In a process of producing leaded steel having a substantially uniform dispersion of finely divided lead particles varying in the amount present throughout major portions of the steel by no more than 0.03 percent by weight, the predominant size of said particles being no greater than about 10 microns in diameter and essentially all of the lead in said steel having been dissolved in the steel when molten, including the steps of adding lead to molten steel in a vessel while the steel is at least 10 degrees Fahrenheit above the liquidus temperature of the steel, providing a slag cover over the steel in the vessel and Stirring the steel after the lead is added, the improvement comprising introducing the lead as at least two substantially discrete additions, each having a lead content greater than one-fifteenth of the total amount of lead added but no greater than 0.35 percent by weight of the molten steel and added lead and spaced apart in time by at least one minute, the total additions together containing an amount of lead sufficient to produce a lead content in the steel greater than 0.15 percent by weight and no greater than 0.26 percent by weight.
 6. A process as set forth in claim 5 wherein the first of said at least two substantially discrete lead additions has a greater lead content than any subsequent addition.
 7. In a process of producing leaded steel having a substantially uniform dispersion of finely divided lead particles essentially all of the lead in said steel having been dissolved in the steel when molten, including the steps of adding lead to molten steel in a vessel while the steel is at least 10 degrees Fahrenheit above the liquidus temperature of the steel, providing a slag cover over the steel in the vessel and stirring the steel after the lead is added, the improvement comprising introducing the lead in at least three and no more than five substantially discrete additions none of which contains lead in an amount exceeding .35 percent by weight of the molten steel and added lead, separating each addition from the next by a time interval at least one minute and no more than fifteen minutes, the total amount of lead added being less than 1 percent by weight of the molten steel and added lead.
 8. A process as set forth in claim 7 wherein the first of said discrete additions has a lead content at least as great as any subsequent lead addition and the time interval between additions is no greater than seven minutes.
 9. In a process of producing leaded steel having a lead content greater than 0.15 percent by weight and no greater than 0.26 percent by weight in the form of a substantially uniform dispersion of finely divided lead particles varying in the amount present throughout major portions of the steel by no more than 0.03 percent by weight, the predominant size of said particles being no greater than about 10 microns in diameter and essentially all of the lead in said steel having been dissolved in the steel when molten, the steps comprising containing molten steel in a vessel at a temperature of between 10 and 135 degrees Fahrenheit above the liquidus temperature of the steel, providing a slag cover over the steel in the vessel, introducing an initial quantity of lead in divided form to the steel and thereafter introducing additional lead to the molten steel, said initial quantity of lead being a greater percentage of the total lead added than any subsequent addition, the total lead added being sufficient to produce a lead content in the steel greater than 0.15 percent by weight and no greater than 0.26 percent by weight, stirring the molten steel in the vessel during and after the addition of lead for a total time period of between five and thirty minutes, removing steel containing lead essentially only in the dissolved state from the vessel, and solidifying the steel removed from the vessel.
 10. A process as set forth in claim 9 wherein the lead is introduced as at least two substantially discrete additions, each discrete addition having a lead content greater than one-fifteenth of the total amount of lead added but not greater than 0.35 percent by weight of the steel and spaced apart in time by at least one minute and not more than 15 minutes, and the total lead added being less than 1 percent by weight of the steel.
 11. A process as set forth in claim 10 wherein the first of said at least two substantially discrete lead additions has a greater lead content than any subsequent addition.
 12. A process of adding lead to steel to assure a uniform distriBution of only finely dispersed lead particles the largest of which are no greater than about 10 microns in diameter, including the steps of first adding lead to steel in a vessel and then removing the leaded steel from the vessel to a mold, such as an ingot mold or a continuous casting mold or the like, wherein the temperature of the steel is at least 10 degrees Fahrenheit above the liquidus temperature of the steel when the lead is added, two to 15 discrete additions of lead are made at time intervals of at least one minute, a cover layer of slag is provided over the steel in the vessel, the steel is stirred within the vessel for at least one minute after each lead addition and for a total time of at least five minutes, and essentially all of the lead in the steel removed from the vessel is in a dissolved state.
 13. A process of continuously casting steel containing a uniform distribution of finely dispersed lead particles the predominant size of which is less than 10 microns in diameter and the largest of which are no greater than 30 microns in diameter, including the steps of: containing molten steel to which lead is to be added in a vessel at a temperature at least 75 degrees Fahrenheit above the liquidus temperature of the steel; adding an initial quantity of lead to the steel in the vessel and subsequently adding an additional quantity, said initial quantity of lead being a greater percentage of the total lead added than any subsequent addition, the total lead added being sufficient to produce a lead content in the steel greater than .15 percent by weight and no greater than 0.26 percent by weight; applying a slag cover to the steel in the vessel; inductively stirring the steel in said vessel for a total time of at least five minutes after the addition of lead; teeming steel containing lead essentially only in a dissolved state from said vessel to a tundish and flowing said steel from the tundish to a continuous casting mold; and continuously casting a solidified shape of leaded steel. 